Current gas turbine engines have peak temperatures in the combustor approaching 3000.degree. F., yet alloys used to construct the turbine engines have melting points in the range of 2200 to 2400.degree. F. Consequently, ample cooling of the turbine engine components is required. Both open-loop and closed loop cooling systems have been developed to provide turbine engine cooling needs.
Both types of systems commonly use pressurized air as a cooling medium. High pressure air provides better heat transfer characteristics than low pressure air as well as providing leakage or film cooling of turbine components. In open-loop cooling systems, spent cooling air passes through the cooled component and into the hot gas path. To pass out through the component, the cooling air must have sufficient pressure to overcome the pressure within the path. Additionally, the pressurization is required for leakage supply, so that air can escape from the component while preventing the hot gas from entering the component. As a result of the need to overcome the surrounding pressure, components closer to the combustor, which experience higher pressures and hotter temperatures, require higher pressure cooling air. Conversely, components further from the combustor require less pressure.
According to conventional air cooling systems, the pressurized air requirements are met by bleeding air out of the compressor stages and feeding that air to the turbine stages. After use in cooling, the spent cooling air combines with the main turbine gas flow and exits the turbine through the exhaust system. To minimize the thermodynamic penalty, open-loop cooling systems remove the air from the lowest possible compression point. An exemplary turbine employing such an open loop system is described in Scalzo et al., A New 150 MW High Efficiency Heavy-Duty Combustion Turbine, ASME Paper No.88-GT-162 (1988). Significantly, in open-loop air cooling systems, spent cooling air is bled into the gas path where it passes out of the turbine engine with the exhaust. Such a design is inefficient because the spent cooling air dilutes the main gas flow thus performing less useful work than it would have had it been heated in the combustion process.
Closed-loop cooling systems have been proposed that use an external compressor to provide the necessary pressure to circulate the coolant and to generate the required heat transfer characteristics. In such a system, air circulates out of the turbine after providing the cooling and is recompressed before being injected directly into the combustor. However, this type of closed-loop cooling system requires expensive external compressors. Applicants have recognized that more efficiency can be gained if the external compressors can be eliminated.
Thus, there is a need for a closed-loop cooling system for use in a turbine engine that recovers the compressed air after it has been used in cooling without the need for external compressors.